This invention relates to imaging systems and more particularly to liquid development systems and liquid developers for use in color reproduction utilizing multiple development.
Color electrophotography with multiple development is capable of producing color reproductions by the following exemplary procedures. A suitable photoconductor such as substantially white zinc oxide photosensitive paper, Electrofax paper for example, is electrostatically uniformly charged in the dark and then exposed through a green filter to an imagewise projection of a color image to form an electrostatic latent image on the photoconductor. The electrostatic latent image is then developed with magenta colored toner to form a magenta colored image corresponding to said electrostatic latent image. The zinc oxide photosensitive paper is again electrostatically uniformly charged in the dark and then exposed through a red filter to an imagewise projection of a colored image in register with said magenta developed image to form a second electrostatic latent image, which second image is developed with cyan colored toner. Similarly, the zinc oxide photosensitive paper is again electrostatically uniformly charged in the dark and then exposed through a blue filter to an imagewise projection of a colored image in register with said magenta and cyan developed images to form a third electrostatic latent image, which is then developed with yellow toner to complete a reproduced color image.
The sequence of exposures through colored filters in this multiple development process may be performed in any suitable sequence other than the green, red and blue sequence recited above. A significant drawback of this multiple development process is that after the formation of the image of the first color and during the second imaging sequence consisting of uniformly charging and imagewise exposing followed by development with tonor of the second color, the zinc oxide photosensitive paper is apt to be electrostatically charged more strongly in the portion where said first colored image is formed in comparison with the other portion where such image does not exist. In addition, the portion of the zinc oxide paper where the first colored image is formed is apt to retain charge in nonimage areas when imagewise exposed to a light pattern which is capable of neutralizing the electrostatic charge in the latter portion. This retained potential, which usually ranges from several volts to several tens of volts, arises from the fact that the ion absorbed by the toner during charging is not neutralized during the imagewise exposure to light. Since the toner usually consists of electrically insulating material the neutralization of the ion for example, held by the toner layer, for material the corona ion generated by corona discharge is hindered. Electroconductive toner cannot be employed in electrophotography with multiple development since the portion of the photoconductor having such toner on its surface during the second and third imaging sequences cannot bear electrostatic charge.
Furthermore, when the electrostatic charge on the first toner layer is not completely neutralized, the toner of second color tends to be improperly deposited onto the first toner layer, giving rise to impure color formation. Similar difficulties also arise in the development with the toner of the third color, and the tendency for improper toner deposition increases as the reflective optical density of the toner image already present is increased. The result of these characteristics is that it is very difficult to obtain color reproduction of satisfactory quality.